So eventually when you manage to log what's happening, and see that the ecu Is jumping cells, due to the afm being affected by the bov return, what are you going to do then? The afm can't be faulty only when lifting off the throttle.
The only two options here are shit tune or an issue with the bov return. The tuner is well respected, and it's pretty hard to do a shit tune on a power fc after I'm sure he has done 100s. So that leaves the bov return.